I'm aware of that and also see the problem, but argue that that's partially because most people just aren't used to this. People's actions depend a lot on what they consider the norm. Take waiters or other service people as an example - in some cultures it's usual to tip them, in others it's unusual or even frowned upon.

You would have to at least have a base minimum purchase price and then accept tips on top.

That's also fine. If the initial price were lower than other comparable games then I assume that more people could be convinced to tip. Or even just if a company is very open about their work / income and dedicated to communicating to their players. I think there's already some companies like that, though perhaps not necessarily the big ones.